Mechanisms of Density-dependent Growth and Survival in Tadpoles of Fowler's Toad, Anaxyrus fowleri: Volume vs. Abundance

Density-dependent growth has ordinarily been studied in aquatic ecosystems by varying the abundance of animals in mesocosms of equal volume. Aside from the unequal sample sizes involved with using this abundance-limited method, confounding factors potentially associated with levels of social interactions may also be introduced. The alternative, volume-limited method, i.e., varying the volume while maintaining equal numbers of animals, can provide a test for the presence of potentially confounding factors. Using tadpoles of Fowler's Toad, Anaxyrus fowleri, we examined the effect of density on growth rate, timing of metamorphosis, and size at metamorphosis in both abundance-limited and volume-limited experiments. We found no difference in tadpole growth rate or timing of metamorphosis between these two methods, but the metamorphs emerging from abundance-limited low density treatments were significantly smaller when compared to those in volume-limited low density treatments. Because toad tadpoles may naturally form social aggregations and schools, this suggests that the actual number of animals present may be important for normal social behavior and optimizing feeding rate. If volume-limited and abundance-limited methods of manipulating density are not equivalent, treatment method may itself be a factor that can differentially affect growth variables.